We have studied the changes in membrane architecture that accompany the acrosome reaction, a secretory event of sperm cells that involves multiple fusions between plasma and acrosomal membranes of the sperm head. We found that the acrosome reaction starts (2-5 min after Ca2+ stimulation) with a dramatic redistribution of membrane proteins on the sperm surface. This lateral movement of membrane proteins is bimodal and leads to formation of three domains on the sperm plasma membrane that mimic the topology of the stable structural domains of the acrosomal membrane. Fusion and fission between the two membranes result in a tubular network of hybrid membranes that is arrested at the equatorial segment of the sperm head. The fenestrated veil of tubules that covers the sperm head is lost later on (5 min to 1 hr after CA2+ stimulation); when the acrosomal reaction is completed the sperm head is limited by the inner acrosomal membrane. We continued the study of the junctional changes that accompany the reversal of the osmotic conditions in toad bladder epithelial cells. Water permeability studies carried out in Bourquet-type chambers showed that osmotic reversal did not damage the capacity of vasopressin to induce increases in the transepithelial transport of water. We have also shown hexagonal lattices (similar to gap junctions) proposed as morphological markers for rotavirus infections, in specimens taken from newborns. Therefore, these lattices cannot be used in the ultrastructure diagnosis of rotavirus infections.